1. Technical Field
The subject matter disclosed herein relates generally to optoelectronic devices, and in particular to light extraction from optoelectronic devices.
2. Discussion of the Background
Light emitting diodes (LEDs) are a class of optoelectronic devices that convert electrical energy into radiant light. Typical LED structures are comprised of doped semiconductor layers placed into contact with each other to create a p-n junction. As electrical current is driven through the junction, electrons and holes recombine with each other and emit photons. The energy in each photon is then determined by the energy difference between electrons and holes in their respective energy bands. The mean energy of the photons (or equivalently, the frequency and wavelength) may therefore be engineered by carefully manipulating the band structure within the junction.
One of the major obstacles against high-efficiency LEDs is the problem of light extraction. Because the index of refraction for so many semiconductor materials is relatively high (e.g., index of refraction n>2.5), the escape cone with respect to the ambient environment is generally very narrow (e.g., an escape cone angle θc<20 degrees). When a ray of light strikes a planar interface beyond the escape cone angle, the result is a phenomenon called total internal reflection (TIR), wherein virtually all incident energy is reflected back down into the substrate. Consequently, a large majority of photons from an LED are needlessly converted into waste heat rather than freely radiated out into the ambient environment. Without proper light extraction techniques integrated into the design, the overall efficiency of a typical LED is limited to the range of only 2-5%.